1. Field of the Invention
The current invention relates to the field of circuits and more particularly to the design and manufacture of transmission line pairs.
2. Description of Related Art
Typically, high-speed integrated circuits are designed and manufactured taking inductance effects into consideration. As electronic components such as amplifiers, oscillators, and mixers are designed to operate at higher and higher frequencies, managing inductance effects may become more important and more difficult. For many applications, it is preferable to reduce the number of external components and manage inductance effects with on-chip elements. Because inductive elements may be difficult to model accurately and may not scale simply in terms of desired inductance and physical layout dimension, transmission line structures may be used instead of pure inductive elements. FIG. 1 illustrates an example of a differential amplifier circuit using a transmission line pair (TLP) 58 in an example of a coupled coplanar strip line (CPS) configuration. In this example, transmission lines 60 and 65 are magnetically coupled. Arrows 62 and 68 indicate the direction of alternating currents (AC) and arrows 61 and 67 indicate the direction of direct currents (DC) that may exist when the differential amplifier circuit is operating. The counter-aligned AC currents form an odd-mode signal on the TLP. In this example, the voltages at a position along the length of the transmission lines 60 and 65 are of opposite sign for an odd-mode signal. The aligned DC currents form an even-mode signal on the TLP. The voltages at a position along the length of the transmission lines 60 and 65 are of the same sign for an even-mode signal. In this example, the differential AC output signal of the amplifier generates an odd-mode signal on the TLP. In this example, considering transmission lines shorter than one quarter of the operational signal wavelength (lambda or xcex) to minimize the possibility of significant line losses and/or complex line behavior over different frequency bands, the net effect of the positive coupling of lines 60 and 65 is to reduce the inductive reactance seen by the differential signal at nodes 59 and 69 and raise the inductive reactance seen by the common mode signal at nodes 59 and 69. In this example, the differential signal may be represented as:
V1xe2x88x92V2
where V1 is output voltage 55 and V2 is output voltage 54.
In this example, the common mode signal may be represented as:
(V1+V2)/2.
Lines 60 and 65 may also be electrostatically coupled. Typically, the mutual capacitance of lines 60 and 65 reduces the inductive reactance seen by both signal modes. Typically, the inductive reactance for the differential signal is reduced to a greater degree than that for the common mode signal.
Typically, the effect of both magnetic and electrostatic coupling of the lines 60 and 65 is therefore to reduce the inductive reactance seen by the differential signal relative to that seen by the common mode signal. It is often desired to have a large inductive reactance seen by the differential signal and a small inductive reactance seen by the common mode signal.
One technique for addressing this issue is to use uncoupled transmission lines or coplanar wave lines (CPW). For uncoupled transmission lines, the inductive reactance is the same for differential and common-mode excitations. For similar cross-sectional line geometries, the absence of coupling between CPW lines tends to result in a higher inductive reactance for the differential signal and a lower inductive reactance for the common-mode signal relative to the CPS lines. CPW lines may be implemented by effectively isolating the transmission lines from each other by introducing a ground plane between the transmission lines. This technique is useful when space is available. However, typical CPW layouts tend not to be as compact as CPS layouts and typically require constructing a larger structure to accommodate a ground plane and associated insulating layers.
Accordingly, it is desirable to have a device capable of producing a high effective inductance per unit layout area for differential signals and a low effective inductance per unit length for common-mode signals with predictable and scalable characteristics. Preferably, this device should support application in high-speed systems over a wide bandwidth and frequency range.
It is an object of the current invention to produce an electronic apparatus with a high inductive reactance for differential signals per unit area with predictable and scalable characteristics. It is also an object of the current invention to produce an electronic apparatus with a small inductive reactance for common-mode signals relative to its inductive reactance for differential signals. This is achieved by creating a TLP in a counter coupled coplanar strip line (CCPS) configuration. A simple TLP in a CCPS configuration may be implemented by aligning two transmission line structures substantially in parallel and coupling the transmission line structures with one or more appropriate electrical signal sources and one or more electrical signal receivers such that when a signal is applied, the currents associated with the differential component of the source signal in the first transmission line structure and the second transmission line structure are aligned and the currents associated with the common mode component of the source signal in the first transmission line structure and the second transmission line structure are counter-aligned. In this case, the differential component of the output signal of the amplifier substantially generates an even mode signal on the TLP. This CCPS configuration tends to establish a high inductive reactance for the differential component of the source signal and a low inductive reactance for the common-mode component of the source signal.
According to a simple embodiment of the current invention, the transmission lines are substantially parallel, substantially straight and implemented in the same plane resulting in a structure with predictable and scalable properties. However, alternate embodiments of the current invention may comprise more complex transmission line structures. For example, various embodiments of the current invention may be implemented partially or wholly in different planes and/or have a large or small radius of curvature and/or be nearly, but not exactly, parallel. According to an alternate embodiment of the current invention, the transmission line structures may have a circular layout configuration. According to a preferred embodiment of the current invention, a high degree of symmetry may be maintained in the transmission line structures. Preferably, the transmission lines structures may comprise substantially straight and parallel lines in the same plane with an out-of-plane crossover structure wherein some portion of the first transmission line structure crosses over, under or around some portion of the second transmission line structure.
Advantageously, the current invention may be designed and built using currently available technology and integrated into a variety of different devices such as, but not limited to, broad-band amplifiers, high-speed logic gates, narrow-band amplifiers, mixers, oscillators, wireless local area networks, satellite communications devices, global positioning systems and high-speed communication systems. Optionally, according to the current invention, a transmission line structure pair in the CCPS configuration may be implemented as an integrated circuit chip as a separate component or integrated with one or more other electronic elements.